Red Conglomerate Hills Near Oudtshoorn, South Africa

Red conglomerate hills with a rock arch.

On Monday I shared a picture of a pretty conglomerate that I recently walked across. Today I thought that I would share a few more pictures of this conglomerate formation and also tell you a little more about it. Although in Monday’s picture the conglomerate looks somewhat dull-colored, the conglomerate formation is actually impressively red in color in many places, so much so that hills made of the conglomerate are known as “the red hills” or “the red stone hills”.

My husband and I hiked amongst the red conglomerate rocks during a recent stay at a wonderful holiday farm known as the Red Stone Hills, named in honor of the local geology, of course! The farm is located a few miles outside of Oudtshoorn, South Africa, a town known for ostrich breeding… so you pass fields of ostriches on your way to the farm! We also stayed at the farm a couple of years ago although I neglected to share pictures of that visit here on Georneys. Around the farm there are several hills of the striking red conglomerate rock. On some of the hills, impressive rock arches have formed. During our previous visit, we hiked up to one of the rock arches and took some pictures. The climb was quite strenuous, however. We only had a few hours for hiking on our recent visit, so we kept mostly to the valleys and observed the rock arches from a distance.

Geologically, the red conglomerates are part of the Buffelskloof Formation. This formation has also been called Enon in the past, so some people still refer to the red rocks as the Enon Conglomerates. The Buffelskloof Formation also contains breccias. The conglomerates and breccias were deposited in a large, land-locked basin in the early Cretaceous. The conglomerates were deposited by rivers and streams while the breccias represent scree deposits. The precise age of the Buffelskloof Formation is not known very well because of poor fossil preservation, most likely due to the sediments being emplaced in an oxidizing environment. Oxidation is also responsible for the reddish color of the rocks. The red color results from oxidation of iron in the matrix of the conglomerate/breccia rocks.

Without further ado, here are some more pictures of the pretty red conglomerates:

The lovely guest cottage where we stayed, nestled at the foot of one of the red conglomerate hills.

Another one of the lovely guest cottages on the farm.

The start to our walk amongst the red conglomerate hills.

Red conglomerate hills in the late afternoon sun. — Red conglomerate hills with intriguing weathering patterns.

Red conglomerate hills in the foreground and distance.

A closer view of the conglomerate texture.

Red conglomerate hills and impressive aloe. — Red conglomerate hills and impressive aloes.

I managed to find my pictures from our last visit to the Red Stone Hills farm, so here are some closer images of one of the impressive rock arches:

Standing in the conglomerate rock arch. For scale, my husband is about 6’2″ tall.

Looking through the arch to more conglomerate hills.

A close-up view of some conglomerate texture near the arch.

One last view of conglomerate texture. — Another view of conglomerate texture.

Conglomerate texture, with husband for scale.

If you ever find yourself near Oudtshoorn, be sure to visit these impressive red conglomerate rocks!

Monday Geology Picture: Conglomerate Near Oudtshoorn, South Africa

Pretty red conglomerate, with my foot for scale. — Pretty conglomerate, with my foot for scale.

A couple of weeks ago I spent part of a weekend staying near Oudtshoorn, South Africa. During a hike, I walked by and, at times, across a beautiful Cretaceous age conglomerate. Here’s one image of the conglomerate, taken when I hiked across it. I’ll share some more pictures in another post.

Mystery Rock #5

I haven’t posted a mystery rock in awhile, and I have some catching up to do!

A little over a month ago I received an email from Liz, who is a Middle School teacher in the Boston area.

Liz wrote,

Once you have a moment and are not too busy with post-wedding stuff, could you take a look at the rock pics I have attached? The 8th grader I work with wants to know where the two parallel white lines came from. Any theories? I would really appreciate it!!

And she sent me the two following pictures:

There’s no scale in the above pictures, but Liz indicated to me by email that the rock is approximately 3 feet across.

First of all, great question Liz (and Liz’s student). However, before I talk about the white lines in the rock and how I think they formed, let me first talk a little bit about the rock itself. My first task is to identify the rock.

Let me work through my basic identification questions:

Is the rock natural or manmade?

Since I can’t see the field context and the rock doesn’t seem to be in situ (i.e. attached to bedrock), there’s a slim chance that this rock is manmade concrete rather than a natural rock, but the morphology and weathering of the rock as well as the presence of the white lines (I’ll explain more in a minute) suggests to me that this is a natural rock. Liz also mentioned to me that there are similar rocks nearby, so perhaps there’s an outcrop in the vicinity.

Is the rock igneous, sedimentary, or metamorphic?

The rock is composed of pieces of other rocks which have been cemented together, so the rock is sedimentary.

So, what is the rock?

The rock appears to be a coarse, poorly-sorted conglomerate. A conglomerate is a type of sedimentary rock that consists of rounded clasts (rock fragments) which have been cemented together. The clasts are generally rounded from transport in water, such as in a stream. The cement composition varies, but is often composed of calcium carbonate or silica-rich material. I’m not sure about the matrix composition for this rock– Liz tried (and failed) to break off a piece of the rock, so it might be a harder silica-rich matrix. However, I know from my work with travertines that calcium carbonates can also be quite hard and difficult to break. A simple way to identify the matrix material is to pour some dilute acid on the rock– if the rock fizzes, the matrix is calcium carbonate. This conglomerate is coarse because it contains fairly large clasts (based on the rock size of ~3 feet). This conglomerate is poorly-sorted because the clasts are of different sizes. That is, some of the clasts are very large and some are very small. Coarse, poorly-sorted conglomerates tend to form in high-energy environments, such as a turbulent streambed. Because of the weathering of the conglomerate, I can’t tell the composition of the clasts– they are likely of variable composition.

Now that I’ve identified the rock, let me move on to the two parallel white lines. Those two white lines are called veins. A vein is a secondary, sheetlike mineral deposit that forms within a pre-existing rock. Those two veins are actually pretty neat! Let me answer some questions about them:

How did the two white veins form?

The two white veins formed when fluids permeated the conglomerate and a secondary mineral was precipitated (fell out of solution). The veins either formed along a pre-existing fracture in the rock or formed when the veins created their own fracture path as the fluids moved along a preferred fluid flowpath.

When did the two white veins form?

Since they cross through the cobbles of the conglomerate (especially the uppermost vein), the two white veins must have formed after the conglomerate was cemented together. Veins actually always form after the primary rock. However, multiple generations of veins can cross-cut each other. Veins are a very useful tool for geologists trying to understand cross-cutting relations— something that helps geologist determine the order in which a sequence of geological deposits were emplaced.

What are the two white veins made of?

My guess is that the veins consist of quartz, but they could also possibly be calcite.

Why are the two white veins parallel to each other?

Well, veins tend to form along weaker or more permeable parts of a rock. Weakness or permeability within a rock is often oriented in a particular direction or directions. This is because of stress (in a particular direction) that a rock has experienced or a result of the rock’s structure, which may be different in different directions. Thus, veins within a rock often form parallel to each other.

I hope the above answers a few questions about the mystery rock. If you have any ideas about this rock, please feel free to add some comments below. Also, if you want to see my previous mystery rock posts, you can find them here.